The present invention relates to an information processing device, in particular, an information processing device which has an electrically erasable programmable read only memory (hereinafter expressed by the acronym "EEPROM") with an error check and correction circuit (hereinafter referred to as "ECC circuit").
The EEPROM installed in an information processing device generally has the disadvantage that, in the course of a repetition of erasing and writing data, a memory cell in the EEPROM is destroyed and a data is lost. In order to cope with this, there has been developed an EEPROM having an ECC circuit. The ECC circuit treats a Hamming code which, for example, comprises data of 8 bits and redundant data of 4 bits added thereto. In this example, even if there is destruction of a memory cell corresponding to one of the 12 bits which is the sum of the data itself of 8 bits and redundant data of 4 bits, it is possible to restore or reconstitute the original data. Thus, the information processing device having such an ECC circuit enjoys high reliability.
However, even for the information processing device which includes the EEPROM having the ECC circuit mentioned above, it is impossible to restore the original data when there are more bits than those which can be corrected by the ECC circuit. In the case of a Hamming code which comprises data of 8 bits and additional redundant data of 4 bits, for example, it is impossible to recover the data when the memory cells corresponding to 2 bits out of the sum of 12 bits are destroyed.
The destruction of the memory bits gradually progresses through the repetition of erasing and writing of data.
In a conventional information processing device which includes the EEPROM having the ECC circuit, assuming that one bit of the memory cells is destroyed but the correct data is being obtained by the ECC circuit, it is impossible to detect the destruction of even a part of the memory cells, so that there is a fear that the destruction of the memory cells further progresses. Hence, the abnormality of the EEPROM cannot be known until the correct data therefrom becomes no longer obtainable. This results in a problem in that the abnormality causes malfunction of the whole information processing device. This problem is serious when there are a great number of repetitions of erasing and writing data.